It is incorrect. "I say" is the right one.
No. This is incorrect. We always say 'India has".
yes.
As "ya" is a slang term, technically it's not grammatically correct to speak using the term. It's an abbreviation of the word "yeah" (which I'm sure you knew already...) and becomes a cut word that...
Say either "her and me" if in the objective case or "she and I" if in the nominative case; never "her and I" which shows inconsistency in case use.